The acid-catalyzed hydrothermal reaction is reported which forms a new carbon-carbon bond in the hydroxy carboxylic acid. It is found that glycolic acid is produced from formaldehyde in hot water at 225 degreesC with HCl. This reaction is a chemical evolution process from a C1 compound to a C2. and does not proceed in the absence of acid. A reaction of 0.3 M (mol/dm(3)) of formaldehyde with the double amount of HCl yielded 60% of the maximum possible amount.